Composition for accelerating the setting of calcined gypsum and the product formed thereby

ABSTRACT

A composition for accelerating the setting of an aqueous calcined gypsum slurry, the composition comprising finely ground calcium sulfate dihydrate and a lignosulfonate. The composition, when added to an aqueous calcined gypsum slurry, accelerates the setting process and shortens the set time period as determined by the temperature rise set test procedure. The composition is useful for making gypsum wallboard.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to the process of setting a calcined gypsum slurry, is more particularly concerned with a composition for the delayed accelerating of the setting of an aqueous calcined gypsum slurry to form gypsum wallboard or plasterboard.

2. Prior Art

Gypsum board or plasterboard has long been a large volume commercial article of commerce. Generally, gypsum board is manufactured by dispersing calcined gypsum in water and adding thereto a lightweight pregenerated foam to control the finished density of the slurry and the subsequent board. Additives conventionally used in minor amounts include accelerators, bond protecting agents, fibrous reinforcements, and consistency reducers. Typical of accelerators are calcium sulfate dihydrate, potassium sulfate, ammonium sulfate, and aluminum sulfate. Bond protecting agents are usually cereal flours or starches. The fibrous reinforcements may be either cellulosic or glass. Consistency reducing agents are typified by the lignosulfonates, of which ammonium lignosulfonate is particularly advantageous. These additives are used in minor quantities in relation to the total weight of the board core, and represent, in total, less than 5%, usually less than 2%, of the weight of the finished core.

The calcined gypsum slurry containing the desired added ingredients is prepared in continuous mixers, as for example a primary-secondary mixer combination described in Lane et al., U.S. Pat. No. 3,359,146, or a multi-pass mixer described in Camp U.S. Pat. No. 2,660,416. The mixed slurry is continuously deposited on a paper cover sheet moving beneath the mixer. A second paper cover sheet is applied thereover and the board is passed under a roll or rolls to determine the thickness. The continuous strip thus formed is conveyed on a belt until the calcined gypsum has set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture.

The most common form of accelerator used for setting a calcined gypsum slurry is calcium sulfate dihydrate. This material appears in many forms many of which can be used when properly prepared by grinding to a high degree of fineness. This material has been found to be an excellent accelerator. However, it has been found that when the material is stored prior to use, it is highly susceptible to becoming calcined, particularly in the presence of calcined gypsum which has been bagged while still warm or in the presence of moisture or heat generally. U.S. Pat. Nos. 2,078,199 and 3,813,312 have disclosed that when sugar or soluble dextrin and ball-milled together with the calcium sulfate dihydrate, the material functions to prevent dehydration of the accelerator, even when subjected to various atmospheric conditions on storage. Additionally, U.S. Pat. No. 3,870,538 has disclosed that starch may be finely ground together with calcium sulfate dihydrate to serve as a preservative of the accelerator. However these materials, particularly sugar, have become very expensive to a degree almost prohibitive of their use.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a novel composition for accelerating the setting reaction of an aqueous calcined gypsum slurry.

It is a further object to provide a composition of the type described which maintains its effectiveness even under varying atmospheric conditions normally encountered during storage.

It is a further object to provide an accelerator composition which is relatively inexpensive.

It is another object to provide an accelerator composition which enhances the strength of the finished board.

It is a still further object of the invention to control the hydration of a calcined gypsum slurry to provide relatively slow temperature rise during the early portions of the hydration period as for example for the first three minutes, followed by a rapid temperature rise toward the end of the hydration period.

It is still another object of the invention to provide a method for preparing gypsum wallboard and to a product prepared by the said method having excellent strength and excellent adhesion between the hydrated gypsum core and the paper cover sheets affixed thereto.

Still further objects and advantages of the invention will appear from the following description.

According to the invention, a composition for accelerating the setting of a calcined gypsum slurry is prepared by finely mixing or grinding together calcium sulfate dihydrate and a lignosulfonate. The resulting accelerator composition may be stored for extended periods and subsequently added to a gypsum slurry. The accelerator composition substantially reduces the hydration period, and results in a gypsum board having excellent properties. The lignosulfonates are inexpensive materials and are commonly used in gypsum slurries to reduce water demand. If desired, the amount of lignosulfonate added to produce the accelerator may be subtracted from the amount used to reduce water demand, since the lignosulfonate added to the accelerator composition may also function to reduce water demand. However, the amount used to form the accelerator is of almost negligible amount and expense, so that a saving is accomplished even without the use of this expediency. Generally from about 0.5 to about 99% lignosulfonate based on total by weight of the accelerator composition may be used, although a preferred range is about 3 to about 25%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lignosulfonates, or lignin sulfonates as they are sometimes known, are anionic polyelectrolytes having molecular weights varying between 1,000 and 20,000. They are obtained as waste products from the sulfite pulping of wood, and are either used in the form obtained or else processed to increase purity and surface activity. The organic structure of the materials has not been completely determined but it is known that the basic lignin monomer unit is a substituted phenylpropane. It can be theorized that a repeating unit of the polymeric lignosulfonate could have the following structure, as shown in Table 1.

                  Table 1                                                          ______________________________________                                          ##STR1##                                                                 

Lignosulfonates are marketed by many companies under various trademarks. A number of lignosulfonate materials are manufactured and marketed by American Can Company, Greenich, Connecticut, and are listed under their trademarks together with their chemical analyses and physical characteristics, as follows in Table 2. The materials are prepared by refining crude lignosulfonate.

                                      Table 2                                      __________________________________________________________________________     TYPICAL ANALYSIS (Moisture-Free Basis)                                                                    Marasperse                                                                           Marasperse                                                                           Marasperse                                                         C-21  CB    N-22                                    __________________________________________________________________________     pH--3% Solution            7.0-8.2                                                                              8.5-9.2                                                                              7.5-8.5                                 Total Sulfur as S. %       6.8   2.6   7.3                                     Sulfate Sulfur as S, %     .3    .1    1.0                                     Sulfite Sulfur as S, %     .09   0     .12                                     CaO, %                     5.2   .03   .55                                     MgO, %                     .3    Trace .3                                      Na.sub.2 O, %              6.1   9.9   13.2                                    Reducing sugars, %         1.5   0     .8                                      OCH.sub.3, %               7.9   12.7  7.7                                     PHYSICAL CHARACTERISTICS                                                                                  Marasperse                                                                           Marasperse                                                                           Marasperse                                                         C-21  CB    N-22                                    __________________________________________________________________________     Usual Form                 Powder                                                                               Powder                                                                               Powder                                  Moisture Content (Max., % H.sub.2 O)                                                                      7.5   8.0   7.0                                     Color                      Brown Black Brown                                   Bulk Density (lbs./cu. ft.)                                                                               35-40 43-47 35-40                                   Solubility in Water (%)    100   100   100                                     Solubility in Oils and Most Organic Solvents (%)                                                          0     0     0                                       Surface Tension, 1% Sol'n (in dynes/cm)                                                                   49.4  51.4  52.8                                    __________________________________________________________________________

The following lignosulfonates are also produced by the American Can Company and are unmodified or partially modified organics derived from the sulfite pulping of wood. These materials contain lignosulfonates of a broad range of molecular size, oligosaccharides, as well as wood sugars or wood sugar derivatives. The materials and their analysis and properties are produced below in Table 3 as obtained from Bulletin No. 131 of American Can Company.

                                      Table 3                                      __________________________________________________________________________     TYPICAL ANALYSIS (Moisture-free Basis)                                                     Norlig                                                                               Norlig                                                                              Norlig                                                                              Norlig                                                                              Norlig                                                                             Norlig                                                                             Norlig                                                                             Norlig                                                                              Norlig                                                                             Norlig                               11    11g  11h  41   41g 41h 11d 11da                                                                               12   21c                      __________________________________________________________________________     pH, 3% solution                                                                            4.0-4.5                                                                              6.0-7.5                                                                             6.0-7.5                                                                             3.0-3.5                                                                             6.0-7.5                                                                            6.0-7.5                                                                            7.5-9.2                                                                            3.5-4.5                                                                            3.0-3.7                                                                             7.3-8.7                  Total Sulfur as S, %                                                                       5.4   5.1  4.9  5.9  5.8 6.1 5.3 5.6 5.5  6.0                      Sulfate Sulfur as S, %                                                                     Trace 0.2  0.2  0.1  0.1 5.7 0.3 0.5 0.2  .3                       CaO, %      6.3   5.8  7.5  5.1  5.7 7.7 12.6                                                                               6.4 0.6  5.6                      MgO, %      0.3   0.3  0.3  0.3  0.2 0.3 0.34                                                                               0.3 0.2  .2                       Na.sub.2 O, %                                                                              --    2.5  --   --   3.3 --  --  Trace                                                                              5.4  4.8                      Reducing Sugars, %                                                                         21.4  18.0 18.0 17.3 16.2                                                                               16.0                                                                               2.0-5.0                                                                            2.0-5.0                                                                            21.0 Less than 4.0            OCH.sub.3, %                                                                               8.8   8.6  8.7  9.3  8.8 8.8 8.2 9.2 8.7  7.8                      PHYSICAL CHARACTERISTICS--Powder Form                                                      Norlig                                                                               Norlig                                                                              Norling                                                                             Norlig                                                                              Norlig                                                                             Norlig                                                                             Norlig                                                                             Norlig                                                                             Norlig                                                                              Norlig                               11    11g  11h  41   41g 41h 11d 11da                                                                               12   21c                      __________________________________________________________________________     Moisture Content                                                                           4-6.5%                                                             Max. % H.sub.2 O                                                                           7.0   --   7.5  6.0  --  --  7.5 7.0 6.5  3.5-6.5                  Color       Brown --   Brown                                                                               Brown                                                                               --  --  Brown                                                                              Brown                                                                              Brown                                                                               Brown                    Bulk Density                                                                   (lbs./cu. ft.)                                                                             33    --   33   36   --  --  32  32  35   33-35                    PHYSICAL CHARACTERISTICS--Liquid Form                                          Solids Content, %                                                                          50-60 50-58                                                                               50-52                                                                               50-58                                                                               50-58                                                                              50-54                                                                              --  --  47   40-50                    Viscosity, Brook-                                                              field at 77° F., cps.                                                               45-350                                                                               47-400                                                                              75-260                                                                              61-450                                                                              72-450                                                                             110-300                                                                            --  --  35   40-200                   __________________________________________________________________________

Other lignosulfonates suitable for use in the present invention are produced by Crown Zellerbach, Chemical Products Division, Camas, Washington, and are listed below in Table 4 together with their physical properties and analyses. The data are listed in the Bulletin entitled "The Orzan Products" page 13.

                                      Table 4                                      __________________________________________________________________________     Physical Properties                                                                       Orzan A                                                                               Orzan AL-50                                                                           Orzan S                                                                               Orzan SL-50                                                                             Orzan P                                                                               Orzan PL-40                                                                            Orzan                  __________________________________________________________________________                                                             AH-3                   Physical form                                                                             powder liquid powder liquid   powder liquid  powder                 Color      lt.brown                                                                              brown  tan    brown    lt.brown                                                                              brown   dk.brown               pH of 25% solution                                                                        4      4      7      7        5      7       2.5                    Water content, %                                                                          6      50     5      50       5      60      4                      Bulk density,                                                                             31            31              20             18                     lbs./cu.ft.                                                                    Specific gravity, 1.22          1.25            1.18                           77° F.                                                                  Viscosity,centipoises                                                                            180           130             190                            77° F.                                                                  32° F.     7800          1800                                           Heat value,btu/lb.                                                                        9100                                                                Solubility in water,%                                                                     completely soluble                           99                     Freezing point    23° F. 14° F.   27° F.                  Components                                                                     Lignin sulfonic                                                                           57            48              51                                    acids                                                                          Reducing sugars                                                                           15            12              12             10                     (as glucose).sup.1                                                             Ash.sup.2  1.5           20              5              1.5                    Alkali-liberated                                                                          3             0.1             2.5            2                      ammonia                                                                        Primary cation                                                                            ammonia                                                                               ammonia                                                                               sodium sodium   ammonia                                                                               ammonia ammonia                Elemental Analyses                                                             Carbon      45.6         41.8                                                  Hydrogen   5.6           5.0                                                   Sulfur     6.4           7.0                                                   Nitrogen   3.7           0.5                                                   Sodium     0.05          5.9                                                   __________________________________________________________________________     .sup.1 The following sugars make up the total sugar content                    Mannose         48%                                                            Glucose         15                                                             Xylose          15                                                             Galactose       10                                                             Arabinose        6                                                             Fructose  less than                                                                            2                                                              P. K. Mulvaney, H. D. Agar, Q P. Peniston and                                  J. L. McCarthy, JACS 73, 1255-7 (1951)                                         .sup.2 Orzan A and Orzan S contain the following additional                    components.                                                                              Orzan A                                                                              Orzan S                                                        Calcium   0.2%  0.2%                                                           Potassium 0.1   0.2                                                            Silicon   0.1   0.02                                                           Magnesium 0.05  0.02                                                           Manganese 0.05  0.01                                                           Tin       0.02  0.01                                                           Vanadium  0.01  0.005                                                          Traces aluminum, iron, chromium, nickel, titanium, boron,                      copper, barium, strontium, silver.                                             __________________________________________________________________________

In addition to the materials listed above, a suitable lignosulfonate is marketed as LIGNISOL by Lignisol Chemicals Ltd., of Canada.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In order to test the efficacies of various commercially available lignosulfonates for use in making gypsum setting accelerator compositions according to the invention, the following tests designated as Examples 1-16, the results of which are shown in Table 5, were carried out.

In carrying out each test, a batch of 500 lbs. of a mixture of landplaster (calcium sulfate dihydrate) and the designated amount of lignosulfonate was ground together in a batch-type ball mill until the particle size was sufficiently reduced so that the mixture exhibited a surface area of at least 5,000 cm² /g, and preferably greater than 10,000 cm² /g, as measured by a Fischer Sub-sieve Sizer. Each accelerator composition was tested in a laboratory procedure developed for comparing the properties of various gypsum setting accelerator compositions. In carrying out the tests, in each case an amount of 180 c.c. of distilled water was placed in a 2-speed Waring Blender operated at high speed. A mixture of 200 g Southard molding plaster and 0.1 g accelerator composition was added to the blender and the lid closed. The slurry was mixed for 7 seconds and then emptied into an insulated plastic foam cup, and temperature vs. time was measured at intervals to obtain a temperature rise set (TRS) curve. From the curve the time at which the temperature stopped rising was determined and recorded as the TRS. Samples of each accelerator composition were then baked at 220° F. for 7 hours and retested in the same manner. The remaining samples of accelerator were exposed for 44 hours at 80° F., at 80% relative humidity and then tested by the same procedure described above. FIG. 5 shows the data obtained from the tests for the various materials tested. The Southard molding plaster used in each case is a very pure highly stable calcium sulfate hemihydrate (stucco). In each case 0.1 g accelerator composition was utilized with 200 g Southard molding plaster. These values were arbitrarily chosen and maintained constant for purposes of the test. In practice in forming plasterboard, other varying proportions of accelerator composition to stucco may be utilized depending on the magnitude of the set period desired.

The properties desired in a good gypsum-setting accelerator composition are, first of all, that the composition be one which delays setting of the gypsum composition slurry to which it is added for about 3 minutes, permitting the composition to be formed into board sheet. It is then desired that setting take place rapidly and reach completion as measured by the above-described procedure within a period of not more than about 20 minutes and preferably not more than 10 minutes, as determined by termination of temperature rise.

                                      TABLE 5                                      __________________________________________________________________________                                                    % Combined                      Accelerator Additive     Temperature Rise Set In Minutes                                                                      Water After                                                  After Baking                                                                            After 44 hrs.                                                                           Baking At                                       % By Weight of                                                                              at 220° F for                                                                    Exposure to                                                                             220° F                                                                          Surface Area            Ex.                                                                               Type         Accelerator                                                                             Fresh                                                                              7 hrs.   80° F, 80%                                                                       7 hrs.  cm.sup.2 /gm            __________________________________________________________________________     1  NORLIG-11    25%      15.16                                                                              15.41    15.50    27.8    14,240                  2  NORLIG-11    10%      15.33                                                                              16.66    15.75    18.2    12,240                  3  NORLIG-11    5%       15.50                                                                              16.25    16.84    15.5    12,837                  4  NORLIG-11    3%       16.00                                                                              17.35    17.00    13.1    11,961                  5  MARASPERSE C-21                                                                             3%       16.00                                                                              18.16    19.25    10.6    12,531                  6  SUGAR        5%       16.16                                                                              16.32    16.84    18.3    13,157                  7  LIGNOSOL     3%       16.50                                                                              23.50    18.67    8.0     13,850                  8  ORZAN-G      3%       16.58                                                                              18.66    17.75    14.3    11,696                  9  NORLIG-11    1%       16.75                                                                              20.25    18.00    9.7     11,961                  10 ORZAN A      3%       17.50                                                                              18.75    19.25    13.4    10,121                  11 NORLIG 21-C  3%       17.75                                                                              20.58    19.90    11.6    11,696                  12 MARASPERSE B-22                                                                             3%       17.58                                                                              18.50    19.25    13.6    11,696                  13 MARASPERSE CBOS-3                                                                           3%       20.16                                                                              23.85    23.25    9.6     10,526                  14 C-211 Starch 10%      16.15                                                                              16.00    18.15                                    15 C-211 Starch 25%      16.35                                                                              16.40    19.45                                    16 C-211 Starch 50%      17.05                                                                              17.30    19.15                                    __________________________________________________________________________

The examples set out in Table 5 above are arranged with the various commercial trademarked lignosulfonates in decreasing order of efficacy, that is, with increasing fresh TRS values. Although all the examples exhibited accelerator properties suitable for commercial use in producing wallboard, those having the shortest TRS values are preferable. Additionally, when comparing various percentages of a particular lignosulfonate (Norlig 11) to landplaster by weight, the 25% composition appeared to have the lowest TRS, and the 1% composition the highest, although all proved to be sufficiently good for use in making plasterboard. Example 6 utilizing an accelerator composition in which conventional suger replaced lignosulfonates, and Examples 14-16 in which C-211 Starch, a commercially available acid-processed starch was used, as disclosed in U.S. Pat. 3,087,538, are provided as comparison examples. As can be seen, both 3 and 5% Norlig 11 accelerator compositions were materially superior to that formed with 5% sugar, or 10-50% starch.

In order to correlate the results shown in Table 5 with practical gypsum board fabrication, gypsum board was produced on standard factory machinery utilizing those accelerator compositions shown in Table 5 which exhibited excellent properties. In carrying out the tests standard factory machinery, methods, and formulations were utilized. The formulations were tailored for the particular size of the equipment. In formulating commercial gypsum slurries, generally a larger amount of accelerator composition in proportion to the stucco is required in order that the temperature rise set will be complete before the formed gypsum wallboard enteres the kiln where it is dried, since once the board enters the kiln and is heated, hydration resulting in setting terminates. Various additives commonly used in making gypsum board were also utilized, but are not all emunerated since they do not affect the action of the accelerator composition. Table 6 below shows the formulations and results obtained from the indicated accelerator composition utilizing the particular percentages listed. In every case gypsum wallboard acceptable for commercial use was obtained.

                                      TABLE 6                                      __________________________________________________________________________                        Example 14                                                                             Example 15                                                                              Example 16 Example 17                      ACCELERATOR ADDITIVE                                                                              3% Norlig 11                                                                           25% Norlig 11                                                                           3% Marasperse C-21                                                                        3% Norlig                       __________________________________________________________________________                                                    11                              Gauging Water                                                                             #/MS'   1069    1069     1069       1069                            Foam Water #/MS'   216     216      205        205                             Total Water                                                                               #/MS'   1285    1285     1274       1274                            Temp. Rise Set                                                                            No. Minutes                                                                            10.8    9.5      10.2       10.5                            Fiber      #/MS'   5.2     5.2      5.2        5.2                             Starch     #/MS'   6.8     6.8      6.8        6.8                             Accelerator                                                                               #/MS'   2.2     2.2      2.4        2.4                             Foaming Agent                                                                             #/MS'   .49     .49      .49        .50                             Foam Air   Cu. Ft./MS'                                                                            13.32   13.32    12.14      12.07                           Foam Weight                                                                               #/Ft.3  12.90   12.90    13.59      13.59                           Wetting Agent                                                                             #MS'    1.7     1.97     1.97       1.97                            Retarder   Oz/MS'  1.0 est 1.0 est  1.0 est    1.0 est                         Dry Stucco Weight                                                                         No./MS' 1487    1583     1564       1584                            __________________________________________________________________________

In order to compare accelerator compositions according to present invention with prior art accelerators, accelerator composition were prepared as described above, one utilizing 3% Norlig 11 and the other utilizing 5% sucrose, a prior art accelerator composition disclosed and claimed in U.S. Pat. No. 3,813,312. The results are shown below in Table 7. In each case commercially acceptable gypsum wallboard was obtained having good strength properties and good paper cover sheet adhesion. Since equally good wallboard was obtained from both accelerator compositions, the economy resulting from utilizing 3% Norlig 11 at 7.5 cents per pound over that of utilizing 5% sucrose at 54 cents per pound can be readily seen.

                  TABLE 7                                                          ______________________________________                                                            Example 18  Example 19                                      ACCELERATOR ADDITIVE                                                                              3% Norlig 11                                                                               5% Sugar                                        ______________________________________                                         Gauging Water                                                                              #/MS'      1099        1099                                        Foam Water   /MS'      215         215                                         Total Water #/MS'      1312        1312                                        Temp. Rise Set                                                                             Minutes    10          10                                          Fiber       #/MS'      6           6                                           Starch      #/MS'      6.2         6.2                                         Accelerator #/MS'      3.5         3.5                                         Foaming Agent                                                                              #MS'       .47         .47                                         Foam Air    Cu. Ft./MS'                                                                               10.97       10.97                                       Foam Weight #Ft.3      12.19       12.19                                       Wetting Agent                                                                              #/MS'      2.9         2.9                                         Retarder    Oz/MS'     2.4         2.4                                         Dry Stucco Weight                                                                          #/MS'      1526        1498                                        ______________________________________                                          *Designates pounds/thousand square feet of 1/2inch board                 

As described above, in carrying out the laboratory tests to determine the relative efficacy of various accelerator compositions in terms of Temperature Rise Set, an amount of 0.1 g of accelerator composition was arbitrarily utilized with 200 g of calcined gypsum, the accelerator composition being 0.05% by weight with respect to the weight of the calcined gypsum. However, where longer setting times can be tolerated, as little as 0.01% by weight of the accelerator composition with respect to the weight of calcined gypsum may be utilized.

In the large scale tests from which the data for Tables 6 and 7 were obtained, approximately 0.15% by weight of accelerator composition with respect to the weight of dry calcined gypsum was utilized. This proportion appears to be a suitable value for general production. However, under particular conditions, as low as 0.05% by weight of accelerator composition to calcined gypsum could be used for factory production. The relative amounts used in factory production could be increased to about 1.5% by weight of accelerator composition to calcined gypsum in order to obtain a more rapid Temperature Rise Set. However, when the amount is increased above 1.5%, no substantial benefit is obtained and additionally the material cost increases. Further, when a proportion greater than 1.5% is utilized, the accelerator composition may reduce the period of low temperature rise which is normally desired for the first 3 minutes, and result in a set before the gypsum slurry has been adequately poured onto the forming belt and shaped to desired form and thickness. Generally a proportion range of about 0.05 to about 1.5% of accelerator composition to calcined gypsum for factory production is suitable, and 0.15 to 1.5% preferred.

In preparing the accelerator compositions according to the invention described above, finely ground landplaster was used. However, if desired other forms of gypsum may be used including by-product gypsum such as that resulting from the citric acid process.

The term #/MS' where used denotes pounds per thousand square feet of 1/2 inch board.

Although in the description of the invention above the accelerator composition of the invention was tested for use in the setting of gypsum slurries, the catalyst may also be used in setting inorganic cementitious mixtures wherever a gypsum accelerator is utilized, as for example, in hydraulic cements such as portland cement.

The accelerator composition of the present invention has several advantages over prior art compositions. The lignosulfonates are extremely inexpensive, and much more so that sugar or starch. They are readily available in large quantities even in periods of severe food shortage when sugar or starch would not be available for commercial processing of gypsum. Further, as shown in the results of the experiments, the efficacy of lignosulfonate-containing accelerator compositions may in some cases be greater than that of conventional materials even when used in smaller amounts. Further, the material is easy to use and does not involve any hazards to the health of factory workers.

It is to be understood that the invention is not to be limited to the exact details of operation or compositions shown and described as obvious modifications and equivalent will be apparent to one skilled in the art. 

We claim:
 1. A dry composition for accelerating the setting reaction of an aqueous slurry of calcium sulfate hemihydrate, comprising finely ground calcium sulfate dihydrate and a lignosulfonate, said lignosulfonate being present in an amount of from about 0.5 to about 99% by weight of said composition.
 2. A composition according to claim 1, wherein said calcium sulfate dihydrate and said lignosulfonate are finely ground together.
 3. A composition according to claim 1, wherein said lignosulfonate has a wood sugar associated therewith.
 4. A composition according to claim 3, wherein the particles of said composition have a surface area of from about 5,000 to about 20,000 square centimeters per gram as determined by a Fisher Sub-sieve Sizer.
 5. A composition according to claim 1, wherein the total amount of said lignosulfonate and associated wood sugar is from about 3 to about 75% by weight of said composition.
 6. A composition according to claim 3, wherein said lignosulfonate is ammonium lignosulfonate.
 7. A composition according to claim 3, wherein said lignosulfonate is sodium lignosulfonate.
 8. A composition according to claim 3, wherein said lignosulfonate is calcium lignosulfonate.
 9. A composition according to claim 3, wherein said lignosulfonate is in the acid form.
 10. A composition according to claim 3, wherein the amount of said lignosulfonate and associated wood sugar is from about 3 to about 75% by weight of said composition.
 11. A composition according to claim 3, wherein said lignosulfonate is in the solid state. 